fix broken panel in the main dashboard

[lucabello]

Issue

Closes #343.

(and fixes the broken ruff usage).

Solution

Use $__rate_interval instead of $__interval.

You can see the panel now has data:

Testing Instructions

charmcraft pack
juju deploy <grafana-charm> <resources> grafana
juju deploy prometheus

juju relate grafana:metrics-endpoint prometheus
juju relate grafana:grafana-source prometheus

Open some pages in Grafana, wait for a few scrapes, then look at the dashboard :)

[lucabello]

The charm now packs; integration tests need to be re-run until they pass 🫠

[lucabello]

Note that I'm disabling the oauth test because they started failing on timeout with no reason (they passed on main and this PR only touched a dashboard file).